The ability of microorganisms to convert sugars to beneficial metabolites including fuels, chemicals, and amino acids has been widely described in the literature in recent years. See, e.g., Alper et al., 2009, Nature Microbiol. Rev. 7: 715-723 and McCourt et al., 2006, Amino Acids 31: 173-210. Recombinant engineering techniques have enabled the creation of microorganisms that express biosynthetic pathways capable of producing a number of useful products, including the commodity chemical, isobutanol.
Isobutanol, also a promising biofuel candidate, has been produced in recombinant microorganisms expressing a heterologous, five-step metabolic pathway (See, e.g., WO/2007/050671 to Donaldson et al., WO/2008/098227 to Liao et al., and WO/2009/103533 to Festel et al.). However, the microorganisms produced to date have fallen short of commercial relevance due to their low performance characteristics, including, for example low productivity, low titer, low yield, and the requirement for oxygen during the fermentation process. Thus, recombinant microorganisms exhibiting increased isobutanol productivity, titers, and/or yields are desirable.
The present inventors have discovered that the production of isobutanol can be improved in existing isobutanol-producing recombinant microorganisms by modifying the expression or activity of one or more transporter proteins. Without being bound to any theory, the present inventors have found that modifying the expression or activity of one or more transporter proteins influences the flux through the isobutanol pathway, and particularly impacts secretion of acetolactate. This is particularly unexpected, as no transporters for acetolactate have been described in yeast and a variety of the transporters described herein have been identified as being involved in the transport of unrelated compounds.
The present invention results from the study of these transporter proteins and shows that the suppression of one or more of these transporter proteins improves the production of isobutanol.